One urgent problem in development of high-speed electric machines is to increase their rotation speed, resulting in a smaller size and mass both of the machines and of the driving mechanisms, while keeping the machine power unaltered.
The rotation speed of high-speed electric machines is raised by increasing the rotor speed.
Rotors of the following types are used for high-speed electric machines:
rotors with laminated magnetic rings and windings; PA1 rotors with solid magnetic rings and salient pole system (no windings); PA1 rotors with smooth solid magnetic rings (no winding); PA1 rotors with smooth solid magnetic rings, with permanent magnets (no windings).
A rotor with a laminated magnetic ring and a winding has the lowest strength leading to its limited use in high-speed electric machines, since high stresses developed by centrifugal forces exerted on the rotor in operation cause it to be rapidly destroyed.
The rotor with a solid magnetic ring with a salient pole system has significant losses (both electromagnetic and, particularly, aerodynamic) which are required to be minimized by evacuation of the cavity of a high-speed electric machine incorporating the rotor, thus considerably impairing the technological efficiency of the machine.
The rotor with a smooth solid magnetic ring is the most rugged of all the rotor types mentioned. This rotor, however, exhibits significant losses causing it to be severely heated, which brings about a sharp decrease in the efficiency of the high-speed electric machine in which it is employed, and hence lower machine power.
The rotor with a smooth solid magnetic ring with permanent magnets is somewhat inferior to the preceding one in its ruggedness. This rotor, however, offers great advantages over the other rotors, being essentially free from electromagnetic losses, and the magnetic flux of the permanent magnets obviates the need to consume the energy for electric machine excitation. As a result, the high-speed electric machines incorporating this rotor have a high efficiency. The high efficiency of the high-speed electric machine gives the advantage of smaller size of the rotor for machines of the same power and rotation speed, i.e. the rotor with permanent magnets will have a smaller size, and hence lower mechanical stresses.
In summary, rotors with a smooth solid magnetic ring are to be preferably used in high-speed electric machines.
Known is a rotor of a high-speed electric machine, comprising a shaft, a smooth solid magnetic ring fitted to the shaft, permanent magnets disposed around the magnetic ring, and a retaining ring fitted over the permanent magnets [V. A. Balagurov, F. F. Galteyev, A. N. Larionov, "Electricheskie mashiny s postojannymi magnitami" ("Electric machines with permanent magnets"), publication of 1964, Moscow, Energy Publishing House, p. 52].
The retaining ring, permanent magnets, and magnetic ring all arranged on the shaft in concentric relation to one another constitute a prestressed system due to pretensioning between the magnetic ring and the permanent magnets as well as between the permanent magnets and the retaining ring. As the rotor is operated, stresses caused by centrifugal forces are added to the stresses developed in its members as a result of pretensioning. These additional stresses are increased with an increase in the rotor speed, the maximum stresses occurring in the retaining ring which is farthest removed from the rotor axis. The tensile forces increase with an increase in the rotation speed of the rotor lower the pretension, and hence, the stress in the retaining ring determined thereby, cause the stresses to be redistributed [the stresses due to centrifugal forces are increased, while those due to the tension are decreased]. The residual tension defined as the difference between the pretensioning force and the tensile forces, with the rotation speed of the rotor held constant, should provide for the torque transfer from the shaft to the permanent magnets.
Therefore, with the rotation speed as large as possible for the particular rotor, the initial allowable stresses in the retaining ring consist of the stresses developed by centrifugal forces and the stresses determined by the minimum residual tension providing for the torque to be transferred from the shaft to the permanent magnets.
As a result, the stresses arising in the rotor, including those determined by the residual tension, impose limitations on the rotation speed of the rotor. Further, in this particular rotor, the minimum residual tension is actually selected so that a significant (e.g. twofold) margin is allowed to ensure reliable operation of the rotor, due to a number of manufacturing errors that manifest themselves especially when the rotor members are fabricated from heterogeneous materials, as well as due to arbitrary temperature-dependent deviations and the like.
An increase in the rotation speed of the rotor due to reduced stresses developed in the retaining ring and determined by the minimum residual tension providing the torque transfer from the shaft to the permanent magnets is obtained in the prototype high-speed electric machine (cf. British Pat. No. 1285966, published in 1972).
This rotor of a high-speed electric machine comprises a shaft, a smooth solid magnetic ring enclosing the shaft, permanent magnets secured to the magnetic ring by a thermoplastic compound, and a retaining ring fitted over the permanent magnets in a prestressed condition. The amount of pretensioning provided is somewhat reduced compared with the aforementioned prior art arrangement by virtue of using the thermoplastic compound.
As the rotor operates, the thermoplastic compound is heated as a result of aerodynamic losses, and its volume is increased.
The increase in the volume of the thermoplastic compound partly compensates various manufacturing errors, random temperature-dependent deviations, etc., making it possible to reduce the minimum residual tension margin down to 1.2-1.5, which in turn allows a decrease in the stresses determined by the residual tension with the consequent increase, to a certain extent, in the rotation speed of the rotor.
Similarly to the previous prior art arrangement, however, in this design, the rotation speed of the rotor is also limited by the stresses in the retaining ring including the stresses determined by the residual tension.